Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink can be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. Once the melted solid ink or the gel ink is ejected onto the image receiving member, the ink returns to a solid, but malleable form, in the case of melted solid ink, and to gelatinous state, in the case of gel ink.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving member to form an ink image. The image receiving member can be a continuous web of recording media, a series of media sheets, or the image receiving member can be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an aperture, usually called a nozzle, in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The magnitude, or voltage level, of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data. A print engine in an inkjet printer processes the image data to identify the inkjets in the printheads of the printer that must be operated to eject a pattern of ink drops at particular locations on the image receiving member to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member with reference to electronic image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads are registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads refers to a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the relative positions of the printheads with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with one another across a width of the image receiving member and that all of the inkjets in the printhead are operational. The presumptions regarding the positions of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads so they better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving member is moving as the imaging surface passes the printhead to receive the ejected ink and “cross-process direction” refers to the direction across the width of the image receiving member, which is perpendicular to the process direction. In order to analyze a printed image, a test pattern needs to be generated in a manner that enables determinations to be made as to whether the inkjets operated to eject ink did, in fact, eject ink and whether the ejected ink landed where the ink would have landed if the printhead was positioned correctly with reference to the image receiving member and the other printheads in the printer. In some printing systems, an image of a printed image is generated by printing the ink image onto media or by printing the ink image on an intermediate surface and then transferring the ink image onto media. In both systems, the media is ejected from the system and then scanned with a flatbed scanner or other known offline imaging device. This method of generating a picture of the printed image suffers from the inability to analyze the printed image in situ and from the inaccuracies imposed by the external scanner. In some printers, a scanner is integrated into the printer and positioned at a location in the printer that enables image data corresponding to the ink image while the image is on media within the printer or while the ink image is on the rotating image receiving member.
These integrated scanners typically include one or more illumination sources and a plurality of optical detectors that receive radiation from the illumination source that has been reflected from the image receiving surface. The radiation from the illumination source is usually visible light, but the radiation can be at or beyond either end of the visible light spectrum. If light is reflected by a white surface, the reflected light has the same spectrum as the illuminating light. In some systems, ink on the imaging surface can absorb a portion of the incident light, which causes the reflected light to have a different spectrum. In addition, some inks may emit radiation in a different wavelength than the illuminating radiation, such as when an ink fluoresces in response to a stimulating radiation. Each optical sensor generates an electrical signal that corresponds to the reflected light received by the detector. The electrical signals from the optical detectors can be converted to digital signals by analog/digital converters and provided as digital image data to an image processor.
The environment in which the image data are generated is not pristine. Several sources of noise exist in this scenario and should be addressed in the registration process. For one, alignment of the printheads can deviate from an expected position significantly, especially when different types of imaging surfaces are used or when printheads are replaced. Additionally, not all inkjets in a printhead remain operational without maintenance. Thus, a need exists to continue to register the printheads before maintenance can recover the missing jets. Also, some inkjets are intermittent, meaning the inkjet may fire sometimes and not at others. Inkjets also may not eject ink perpendicularly with respect to the face of the printhead. These off-angle ink drops land at locations other than where they are expected to land. The image receiving member also contributes noise. Specifically, image data corresponding to structure in the image receiving surface and/or colored contaminants in the image receiving surface can be mistakenly identified as ink drops and lightly colored inks and weakly performing inkjets provide ink drops that contrast less starkly with the image receiving member than darkly colored inks or ink drops formed with an appropriate ink drop mass.
In existing systems, the registration process includes two phases referred to as coarse registration and fine registration. The coarse registration process moves multiple printheads in the printer into alignment with a relatively large tolerance. The fine registration process begins after completion of the coarse registration process and performs further adjustments to the registration of the printheads to complete the calibration of the printheads. In existing embodiments, the printed patterns generated in the coarse registration process lack the resolution needed to register the printheads in the printer with sufficient precision to produce the highest quality printed images. Additionally, a fine registration process is incapable of identifying and correcting printhead registration errors that exceed a maximum registration error size covered by the fine registration process. A printer that performs both the coarse and fine registration processes consumes time as well as other resources, such as ink and paper, when performing both the coarse and fine registration processes. Consequently, improvements to the printhead registration process that reduce the time and resources consumed to calibrate the printheads would be beneficial.